1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a method of driving a backlight unit and a method of driving a backlight unit of a liquid crystal display (LCD) device.
2. Description of the Related Art
Presently, LCD devices are being developed as the display devices of next generation because of their light weight, thin profile, and low power consumption, and are adopted in a notebook computer, office automation (OA) equipments, audio/video equipments and the like.
In general, an LCD device is a non-emissive display device that displays images using a refractive index difference and optical anisotropy properties of a liquid crystal material interposed between two substrates.
Among the various type of LCD devices commonly used, active matrix LCD (AM-LCD) devices have been developed because of their high resolution and superiority in displaying moving images. The AM-LCD device includes a thin film transistor (TFT) in each pixel region as a switching device, a pixel electrode in each pixel region, and a common electrode.
FIG. 1 is a block diagram of an LCD device according to the related art. FIG. 2 is a schematic view of an LCD panel according to the related art. In FIG. 1, an LCD includes an LCD panel 2 and a driving circuit unit (not shown) connected to the LCD panel 2, wherein the driving circuit unit is disposed in a periphery of the LCD panel 2. A data signal for red, green and blue colors and a control signal such as an input clock, a horizontal synchronizing signal, a vertical synchronizing signal, a data enable signal and the like is generated from a driving system such as a personal computer to the drive circuit unit. An interface 10 transmits the mentioned data signal and the control signal to a timing controller 12. For example, a low voltage differential signal (LVDS) interface and a telescope technologies limited (TTL) interface are utilized as a typical interface. In the alternative, the timing controller 12 may be formed as a single chip including function of the interface 10.
In FIGS. 1 and 2, the LCD panel 2 includes a plurality of gate lines GL1 to GLn (n is a positive integral) and a plurality of data lines DL1 to DLm (m is a positive integral) crossing the gate lines GL1 to GLn to define a plurality of pixel regions (not shown).
The timing controller 12 generates a control signal for driving a gate driver 20 connected to the gate lines GL1 to GLn and a data driver 18 connected to the data lines DL1 to DLm, wherein the gate driver 20 includes a plurality of gate integrated circuits using the control signal. In addition, the data and control signal is applied to an interface 10, and the interface 10 transmits the data and control signal to the data driver 18.
A reference voltage generator 16 generates a reference voltage of a Digital to Analog Converter (DAC) utilized in the data driver 18, wherein the reference voltage generator 16 is predetermined by a manufacturer with respect to transmittance versus voltage of an LCD panel 2. The data driver 18 responses to the control signals from the timing controller 12 and selects the reference voltage for converting as analog video data. Furthermore, an analog video data generated by the selected reference voltage is supplied to the LCD panel 2, thereby controlling a rotation angle of the liquid crystal molecules.
The gate driver 20 responses to the control signals from the timing controller 12, thereby controlling ON/OFF of a plurality of thin film transistors TFT formed in the LCD panel 2. Here, the gate lines GL1 to GLn are sequentially enabled by the horizontal synchronizing signal and the thin film transistors TFT are sequentially driven by the gate lines GL1 to GLn. Then a plurality of analog image signals from the data driver 18 are applied to the pixels connected to the thin film transistors TFT, wherein each of the pixels corresponds to one of the thin film transistors TFT. In addition, a power supply voltage generator 14 provides a power to respective elements and a common voltage to the LCD panel 2.
Although not shown, a backlight unit, light source, is disposed under the LCD panel 2. Particularly, a direct type backlight unit having a plurality of lamps is utilized when the LCD panel is a large size panel. The direct type backlight unit does not include a light guide plate that changes line light into plane light, but includes the lamps, a reflective sheet preventing light loss where light emitted from lamp is reflected to an image display surface, and light scattering means including a diffusion plate and a plurality of diffusion sheets that emit a uniform light by scattering light toward a top portion of the lamps.
FIG. 3 is a schematic perspective view showing a direct type backlight unit for an LCD device according to the related art. In FIG. 3, a backlight unit includes a plurality of lamps 1, a bottom case 3 fixing and supporting the plurality of lamps 1, and light scattering means 5a, 5b and 5c arranged between the lamps 1 and the LCD panel (not shown), wherein the light scattering means 5a, 5b and 5c provide a light source having a uniform luminosity distribution and prevent image defect, wherein the light scattering means 5a, 5b and 5c may further include a plurality of diffusion sheets and a diffusion plate to increase a light scattering effect.
A reflector 7 is arranged on an inner surface of the bottom case 3 in order to effectively provide light from the lamp 1 to the LCD panel. The lamp 1 includes a cold fluorescent lamp (CCFL), wherein the lamp 1 includes a glass tube, electrodes (not shown) disposed on both end portions of the glass tube. The lamp 1 emits light when power is applied to the electrodes and both end portions of the lamp 1 are fixed in a groove formed in both sides of the bottom case 3.
FIG. 4 is a schematic cross-sectional view of a direct type backlight unit for an LCD device of FIG. 3 according to the related art. In FIG. 4, a direct type backlight unit includes a plurality of lamps 1 parallel with each other along the same direction, a bottom case 3 supporting the lamps 1, a diffusion plate 5a connected to the bottom case 3, and a plurality of optical sheets 5b and 5c over the diffusion plate 5a. More specifically, an end portion of the diffusion plate 5a contacts the bottom case 3. The direct type backlight unit is driven by a scanning driving method such that the respective lamps 1 is sequentially turned ON/OFF in accordance with a frame driving of the LCD panel in order to reduce a power consumption.
FIG. 5 is a timing chart illustrating a driving method of a direct type backlight unit with respect to one lamp according to the related art. In FIG. 5, a timing chart shows that respective lamps is sequentially turned ON/OFF in accordance with a scanning ON/OFF period of a thin film transistor of a gate driver, wherein ON/OFF is performed during one frame period of the LCD panel. In other words, the scanning driving type backlight unit sequentially emits light by supplying concentrated light with respect to only a predetermined region of the LCD panel. For example, when the LCD panel with Extended Graphics Array (XGA; 1024×768) resolution includes eight lamps L1-L8, one of the eight lamps L1 to L8 may control about one hundred of pixel lines.
FIG. 6 is a timing chart illustrating a driving method of one lamp for a direct type backlight unit with respect to one lamp according to the related art. In FIG. 6, since each one of the eight lamps L1 to L8 (of FIG. 5) controls about one hundred of pixel lines in the XGA type LCD panel, the lamp brightness LB in a top position TP, a center position CP and a bottom position BP with respect to the one hundred of pixel lines is the same as each other. However, the faster the liquid crystal response TL driven by synchronizing with a gate signal of the gate driver that goes down from the top position TP to the bottom position BP, the less the driving time in accordance with scanning driving of the gate line is delayed.
Further, since the direct type backlight unit is repeatedly turned ON/OFF, the panel brightness is lower than another driving type such that the lamp is always in ON state. To overcome this problem, a method of driving is suggested such that the lamp brightness increases by increasing a lamp current applied to the lamp.
FIG. 7 is a timing chart illustrating a driving method of a direct type backlight unit with respect to one lamp according to the related art. In FIG. 7, a higher lamp current is applied to the lamp to increase brightness, thereby increasing the panel brightness. However, when the lamp is driven by applying a high lamp current, the high lamp current is applied before the liquid crystal layer has completely responded, as shown in region A, thereby accruing unnecessary power consumption. In addition, when high brightness is obtained after response of the liquid crystal layer is reduced, as shown in region B, the blur reduction resulting from the scanning type backlight unit is depressed.